A total of 218 yearling Hereford, Angus, Santa Gertrudis and Brahman bulls was evaluated for semen traits and testieular volume at average ages of 16 and 20 months in April and August, respectively, during 2 successive years. Brahman bulls reached puberty at a later age than bulls of the Santa Gertrudis, Hereford and Angus breeds. Santa Gertrudis bulls had the largest testes with the Brahman having the smallest during this time span. During the summer months, when the bulls were 16 to 20 months of age, testes size, sperm motility and sperm cell concentration of the ejaculate increased markedly in Brahman and Santa Gertrudis bulls. Angus bulls exhibited stable to slight increases in testes size, semen volume and semen quality, suggesting that this breed had reached its most rapid stage of development by 16 months of age. During this same period Hereford bulls showed a decline in testicular size and semen quality suggesting that Hereford bulls suffered from the environmental stresses of the semitropical summer in Florida. Within the Hereford breed, bulls from the lines of Montana origin exhibited a greater decline than those from the line originating in Florida. (

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